A hybrid powertrain for a vehicle includes two power sources that are operatively connectable to a drive wheel to propel the vehicle. Typically, one of the power sources is an engine and another of the power sources is an electric motor. The engine converts the chemical energy in fuel to mechanical energy through the process of combustion. The electric motor converts electrical energy from a battery to mechanical energy. The battery may be recharged by a generator driven by the engine, an offboard electrical power source (such as the electric grid), etc. The generator may also transmit electrical energy directly to the electric motor.
In a series hybrid-electric powertrain, there is no mechanical connection from the engine to the drive wheel; the engine is operatively connected to the drive wheel via the generator and the electric motor. In a parallel hybrid-electric powertrain, the engine is mechanically connectable to the drive wheel, such as via a transmission and differential.